A liquid crystal display panel includes a substrate provided with drive elements for driving liquid crystal as well as a substrate provided with opposed electrodes opposed thereto, and these substrates are adhered together with their main surfaces opposed to each other with a space of several micrometers therebetween.
FIG. 11 is a schematic cross section of a liquid crystal display panel according to a prior art. For the sake of simplicity, FIG. 11 does not show drive elements, opposed electrodes and alignment films formed on main surfaces of substrates. A sealed space surrounded by two substrates 1a and 1b and a seal member 2 is filled with liquid crystal 6. Columnar spacers 5 arranged in this space define a distance between substrates 1a and 1b. 
For manufacturing the liquid crystal display panel, the two substrates must be adhered together by the seal member with main surfaces thereof opposed to each other, and a region surrounded by the two substrates and the seal member must be filled with the liquid crystal.
A vacuum filling method has been known as one of methods of filling a space with liquid crystal according to a prior art. In this method, two substrates are first adhered by a seal member together with their main surfaces opposed together while applying a pressure thereto. The seal member has an annular form, and is provided at a portion thereof with an opening. When a distance between the two substrates attains a predetermined value, the seal member is cured. Then, the substrates adhered together are cut into sizes of a predetermined display panel. The substrates thus cut are arranged inside a vacuum container, and a vacuum is produced in the vacuum container to produce a vacuum in the space between the substrates. After sufficiently exhausting a gas, the liquid crystal is brought into contact with the opening at the seal member, and the vacuum container is opened to an atmospheric pressure. The liquid crystal is supplied into the space between the substrate by the pressure difference between the pressure in the space between the substrates and the atmospheric pressure as well as the interfacial tension. After a predetermined quantity of liquid crystal is supplied, the opening at the seal member is sealed to store sealingly the liquid crystal. This method of sealingly storing the liquid crystal in the vacuum filling method suffers from a problem that the filling time increases with the size of the liquid crystal display panel.
In recent years, therefore, a method called a “drop adhering method” has been performed for sealingly storing the liquid crystal (see, e.g., Japanese Patent Laying-Open No. 2001-281678). In the drop adhering method, drive elements, opposed electrodes and others are first formed on each of two substrates. Also, spacers for fixing a distance between the substrates are arranged on one of the substrates. Further, an annular seal member(s) for adhering the two substrates is arranged on one or both of the substrates. In this operation, the seal member is not provided with an opening, and is arranged in a closed annular form. Drops of a predetermined quantity of liquid crystal are put on one of the substrates. The two substrates are adhered together in a vacuum with a high position accuracy, and then are exposed to the atmospheric pressure. Thereafter, the seal member is cured to store sealingly the liquid crystal between the two substrates.
In the drop adhering method, the space surrounded by the two substrates and the seal member is closed by performing the adhesion. Since this adhesion is performed in a vacuum, air does not mix into the space filled with the liquid crystal, and only the liquid crystal is present therein. Therefore, by exposing the two substrates to the atmospheric pressure after adhering them in the vacuum, the two substrates are entirely and uniformly compressed together by the atmospheric pressure. The seal member is compressed and collapsed to a predetermined thickness.
The spacers define the distance between the two substrates. The prior art has employed spherical plastic beads or the like as the spacers. In the structure employing the plastic beads, however, the liquid crystal material is not present in positions where the plastic beads are present, and the alignment does not occur in such positions so that leakage of back light, i.e., so-called “light leakage” occurs. In recent years, therefore, spacers of a columnar form (which will be referred to as “columnar spacers” in the invention) are formed as spacers on a substrate for adjusting a distance between the substrates (see, e.g., Japanese Patent Laying-Open No. 2003-131238). The columnar spacers are arranged in a region where interconnections are formed between picture elements, so that the light leakage can be prevented. Further, these spacers are resistant to collapse in the direction of thickness of the substrate, and can offer an advantage that irregularities do not occur in display even when a display screen of the liquid crystal display panel is pressed by a finger or the like.
In the conventional liquid crystal panel, spherical spacers such as plastic beads are dispersed on one of the substrates before adhering the substrates together for keeping a constant thickness of a liquid crystal layer between a TFT (Thin Film Transistor) substrate and a color filter substrate. However, this manner suffers from a problem of occurrence of irregular display due to irregular dispersion or movement of the beads.
For overcoming the above problem, a technique of forming column-structure spacers on a substrate has been developed. The column-structure spacers are formed by applying photosensitive resin onto the substrate and patterning the photosensitive resin by a photolithography method. The column-structure spacers can be formed at desired positions on the substrate surface, and do not move on the substrate surface so that irregular display does not occur. Further, the height thereof can be freely determined depending on manufacturing conditions. However, the liquid crystal material thermally extends in a high-temperature state so that the column-structure spacers cause irregular display due to nonuniformity in cell gap on the surface.
Japanese Patent Laying-Open No. 2001-147437 has disclosed that the irregular display due to changes in temperature can be prevented by storing an elastic energy in the columnar spacers. Each of Japanese Patent Laying-Open Nos. 2003-121857 and 2003-131238 has disclosed a structure in which two or more kinds of spacers having different heights or sectional areas are used for preventing the irregular display even when the liquid crystal material shrinks in a low temperature environment or an excessive load is applied thereto. Japanese Patent Laying-Open No. 2002-229040 has disclosed a structure in which each columnar spacer has a concave or flat top portion for preventing a display failure due to local irregularities in cell thickness.
However, the spacer disclosed in the above patent reference has a high aspect ratio, and therefore is less resistant to elastic deformation so that the spacers may be damaged in a step of rubbing an alignment film, and may not function as the spacers.
In the method (which will be referred to as a “liquid crystal drop adhering method” hereinafter) of performing adhesion after putting drops of the liquid crystal material on the substrate surface, the thickness (cell gap) of the liquid crystal layer depends on the drop quantity of the liquid crystal material. Therefore, when an imbalance occurs between the height of resin spacer and the drop quantity of liquid crystal, an error occurs in display. More specifically, when a large drop quantity of liquid crystal is used, excessive liquid crystal causes irregular display. When the quantity is small, vacuum bubbles occur to cause a significant disadvantage. These vacuum bubbles often occur particularly during an operation of adhering the substrates or at a low temperature, and early overcoming of this problem has been desired.
For overcoming the problem, Japanese Patent Laying-Open No. 2001-281678 has disclosed a method in which a column height of a columnar spacer is measured, and a drop quantity of liquid crystal is controlled based on the measured value. However, when consideration is given to measurement errors, accuracy in control of drop quantity and changes in temperature, the method disclosed in Japanese Patent Laying-Open No. 2001-281678 is not sufficient for the above purpose.
Patent reference 1: Japanese Patent Laying-Open No. 2001-281678
Patent reference 2: Japanese Patent Laying-Open No. 2003-131238
Patent reference 3: Japanese Patent Laying-Open No. 2001-147437
Patent reference 4: Japanese Patent Laying-Open No. 2003-121857
Patent reference 5: Japanese Patent Laying-Open No. 2002-229040